1. Field of the Invention
The present invention relates to a surface-acoustic-wave device (hereinafter, referred to as a SAW device). In particular, the present invention relates to a SAW device having a SAW element on which interdigital electrodes are formed and which is covered with a conductive thin-film to shield the SAW element against an external electric field.
2. Description of the Related Art
FIG. 1 represents an example of construction of a SAW device currently in use.
Interdigital electrodes 2 and I/O (input/output) electrodes 3-1, 3-2 are formed of aluminum (Al) thin films on one surface of a SAW element 1. SAW element 1 is formed on ceramic substrate 11 with interposition of ground pattern 8. I/O terminals 10-1 and 10-2 are formed on ceramic substrate 11.
I/O electrode 4-1 is connected with I/O terminal 10-1 via through-hole 9-1. Similarly, I/O electrode 4-2 is connected with I/O terminal 10-2 via through-hole 9-2.
I/O electrodes 3-1, 3-2 formed on the surface of SAW element 1, on which interdigital electrodes 2 are formed, are connected with I/O electrode 4-1, 4-2 by bonding wire 5-2, 5-2, respectively.
Seam-ring 6 and metal cover 7 define a space above the surface on which interdigital electrodes of the SAW device are formed.
FIG. 2 shows another example of a SAW device in which metal bumps 21-1, 21-2 are employed to perform an electric I/O between SAW element 1 and an external circuit.
In the SAW device shown in FIG. 2, interdigital electrodes 2 and I/O electrodes 3-1, 3-2 are formed on one surface (a first surface) of SAW element 1. The I/O electrode 3-1 of SAW element 1 is connected with I/O electrode 4-1 through metal bump 21-1. I/O electrode 4-1 is formed on the surface of ceramic substrate 11. Similarly, the I/O electrode 3-2 is connected with I/O electrode 4-2, formed on the surface of ceramic substrate 11, through metal bump 21-2.
A ground pattern 8 is formed on one of the surfaces of ceramic substrate 11 where I/O electrodes 4-1, 4-2 are provided. I/O terminals 10-1, 10-2 are formed on the opposite surface of the substrate.
I/O terminal 10-1 and I/O electrode 4-1 are connected through through-hole 9-1, and, similarly, I/O terminal 10-2 and I/O electrode 4-2 are connected through through-hole 9-2.
In this configuration of the device, SAW element 1 is surrounded with seam-ring 6 and metal cover 7.
Either of the SAW devices shown in FIG. 1 and FIG. 2, however, requires an electric-shield chamber made up of ceramic substrate 11, seam-ring 6, metal cover 7, etc. to shield SAW element 1, interdigital electrodes and I/O electrodes 3-1, 3-2 against external electric field.
A problem has been that this requirement of the electric-shield chamber entails high cost for the SAW device. Another problem has been that provision of the electric-shield chamber causes to increase the volume of the SAW device for mounting the SAW element.
Many electric companies have investigated methods of mounting a SAW element directly on a mounting substrate.
This mounting method includes a problem concerning the way of preventing an erosion of an Al pattern formed on a surface of a SAW element (the pattern of the interdigital electrodes). The method includes another problem to be taken into account concerning the way of electric-shielding of the SAW element against an external electric field neither giving rise to a high cost nor enlarging the volume of the SAW device.
In order to solve the former problem, it is advantageous to apply a protective layer (for example SiO.sub.2) on the Al pattern or to place the Al pattern in an airtight arrangement.
Concerning the latter problem, however, no effective method has been currently found.